The present invention relates to improvements in color television receivers and more particularly, to an improved voltage controlled crystal oscillator suitable to be provided in the phase lock loop system of the subcarrier regeneration stage thereof.
In the manufacture of electronic devices such as television receivers, it is desirable to utilize solid-state components to the greatest extent possible in order to realize the advantages inherent in such components.
In accordance with present NTSC standards governing television transmission signals luminance information, representing brightness variations in the reproduced televised image is received on an amplitude modulated main carrier signal. The chrominance information, representing color hue is received on an amplitude modulated 3.58 megahertz subcarrier signal. As is known in the art, the chrominance signal is demodulated in a chrominance demodulator stage such that color control signals are established and applied to the television picture tube thereby reproducing the transmitted image which has correct hue and color characteristics.
The aforementioned synchronous demodulators which respond to the chrominance signal require for their operation; a reference signal which is phase and frequency synchronized to the modulated subcarrier component of the chrominance signal, utilizing reference burst of the subcarrier component which is transmitted at reoccuring time intervals during the transmitted signal for purposes of synchronization. In some prior art circuits the chrominance signal is applied to a phase lock loop system comprising an automatic phase control (APC) detector, and a stable oscillator circuit. One output of the aforementioned oscillator is provided to a hue control circuit which provides a synchronous output signal to the chrominance demodulator network having correct phase information to thereby demodulate the chrominance signal through a matrix, as is understood in the art, which ensures the correct color signals will be supplied to the television tube. As is understood in the art, the output of the oscillator is also provided to a automatic chrominance control (ACC) detector.
One difficulty encountered in prior art demodulator systems relates to the oscillator which is used for providing the 3.58 reference signal in response to the reference sync burst. One particular problem associated with the crystal control oscillator utilized in some prior art systems is that it will occasionally oscillate at the third overtone of the fundamental frequency of the crystal. If the oscillator should oscillate at the third overtone frequency of the crystal, in some television sets a signal will be generated by automatic chrominance control detector for inhibiting the color signal. Hence the viewer would be restricted to seeing a black and white picture. To prevent oscillation at the third overtone frequency, prior art oscillators sometimes have discrete components forming a parallel resonant circuit which has a low impedance at the third overtone frequency. Thus, a trap is provided for the third overtone frequency preventing oscillation to be sustained thereat, as is known in the art. However, because the trap circuit adds components to the television receiver, increased cost are incurred which are borne by the consumer.
Therefore, a need exists to develop a voltage controlled crystal oscillator providing a solution to the problem of overcoming third overtone oscillation which seriously affects the stability and operation of a color television receiver without the need for discrete components which increase production costs for the television receiver.